Seismic Interpretation and Structural Mapping

Core Idea

Seismic interpretation converts migrated seismic images into geological models by identifying reflectors with subsurface interfaces, tracing faults and stratigraphic units, and estimating depths using velocity models. Interpreters map subsurface geology, identify structural traps for hydrocarbon accumulation, assess resource potential, and guide drilling decisions.

Explainer

From your study of seismic migration, you know how raw seismic reflection data is processed to produce an image where reflectors appear at their true subsurface positions. Seismic interpretation is the next step: translating that processed image into a geological model — identifying what each reflector represents, where faults cut through the section, and what the three-dimensional structure of the subsurface looks like.

A migrated seismic section displays reflections as a series of light and dark bands. Each band corresponds to an acoustic impedance contrast — a boundary where rock density or seismic velocity changes abruptly. The interpreter's first task is to correlate these reflections with known geology, typically by tying the seismic data to well logs from boreholes where the actual rock types and depths are known. A strong, continuous reflector at a certain depth might correspond to the top of a limestone formation; a weaker, discontinuous one might mark a sandstone-shale interface. This process of horizon picking — tracing a specific reflector across the seismic volume — builds a map of each geological surface.

Fault identification requires recognizing characteristic patterns: abrupt termination or offset of reflectors, changes in dip, and zones of chaotic or diminished reflections where the rock has been fractured. Normal faults show hanging-wall reflectors dropped down relative to the footwall. Reverse and thrust faults show repeated or stacked reflector packages. Strike-slip faults may appear as subtle lateral discontinuities that are easier to see on horizontal time slices through 3D seismic volumes. The interpreter traces each fault surface through the data, building a structural framework that divides the subsurface into discrete fault blocks.

With horizons and faults mapped, the interpreter constructs structural maps — contour maps of each geological surface showing its depth (or time) across the survey area. These maps reveal anticlines, synclines, fault-bounded closures, and unconformities. In hydrocarbon exploration, the primary goal is identifying structural traps: configurations where an impermeable seal rock overlies a porous reservoir rock in a geometry (such as a four-way dip closure or a fault-sealed compartment) that could trap migrating oil or gas. The interpreter must also convert from seismic two-way travel time to true depth using velocity models, since the same time interval can represent different thicknesses depending on the velocity of the intervening rock. The final product — an integrated structural and stratigraphic model — guides decisions about where to drill, what to expect at depth, and how much resource a prospect might contain.